Optical methods for testing of fluid flow are usually referred as non-intrusive or non-disturbing. Although modern optics is rapidly developing, new methods are rather sophisticated and cannot be easily adopted, which sometimes lead to enormous sum of money paid for technical equipment. There are various methods on how to study the interaction of light with fluid flow, one of which is the Schlieren method. Schlieren systems are based on the refraction of light rays when travel from one medium to another. The schlieren system used in this paper is called the Positive-Negative Grid schlieren system, a modification of R. Burton's schlieren system (1948). Simple relationships between density, temperature and light are investigated. Based on the relationships, a mathematical model for converting schliere data into temperature distribution can be constructed. This paper focuses on the thermal properties that could be extracted from schlieren images, using the model proposed. Problems with obtaining accurate temperature data from schlieren images are normally due to the noise in the images and heat radiation from fluid flow, which alters the schliere data slightly.